Using "a" or "an" with initialisms starting with consonants that sound like vowels

[health professional]

In writing using initialisms starting with "S" do you use "a" or "an". In particular, I am talking about an initialism like STI or STD. Do you write "an STI" or "a STI"?

[Ouisch]

You would say "an" STD. If the phonetic pronunciation of a letter begins with a vowel sound (such as M [em], S [ess], N [en], etc) then "an" is used.

[Casiopea]

Welcome, health professional.